Device for controlling the reactivity of nuclear reactors

ABSTRACT

Device for controlling the reactivity of a nuclear reactor cooled by light water and including a reactor core formed of fuel elements, includes a tube system passing through the reactor core and traversible by a water-soluble neutron absorber of adjustable concentration, separated by the tube system from light water coolant in the reactor core, the tube system including a plurality of substantially vertically extending fingerlike tubes always fully inserted in the fuel elements; a spiderlike holder holding the fingerlike tubes together at the upper ends thereof and serving as distributor for the neutron absorber to the fingerlike tubes, and a holder rod extending from the spiderlike distributor and serving as a central supply line for the neutron absorber to the spiderlike distributor.

States ate DEVICE FOR CONTROLLING THE REACTIVITY OF NUCLEAR REACTORSWolfgang Braun, Buckenhof, Gennany Siemens Aktiengesellschaft, Berlin,Germany Filed: May 7, 1970 Appl. No.: 35,312

Inventor:

Assignee:

lint. Cl. G2lc 7/08, G216 7/22 Field of Search ..176/86 R, 86 LReferences Cited UNITED STATES PATENTS 12/1968 Galli de Paratesi et al..176/86 L 7/1970 French et a] ..176/86 FOREIGN PATENTS OR APPLICATIONS1,081,214 8/l967 Great Britain ..176/86 L Primary Examiner-Carl D.Quarforth Assistant Examiner-Harvey E. Behrend Attorney-Curt M. Avery,Arthur E. Wilfond, Herbert L. Lerner and Daniel J. Tick [5 7] ABSTRACTDevice for controlling the reactivity of a nuclear reactor cooled bylight water and including a reactor core formed of fuel elements,includes a tube system passing through the reactor core and traversibleby a water-soluble neutron absorber of adjustable concentration,separated by the tube system from light water coolant in the reactorcore, the tube system including a plurality of substantially verticallyextending fingerlike tubes always fully inserted in the fuel elements; aspiderlike holder holding the fingerlike tubes together at the upperends thereof and serving as distributor for the neutron absorber to thefingerlike tubes, and a holder rod extending from the spiderlikedistributor and sewing as a central supply line for the neutron absorberto the spiderlike distributor.

15 Claims, 6 Drawing Figures PATENTEDMAR28 I972 SHEET 2 BF 3 XXX Fig.3

DEVICE FOR CONTROLLING THE REACTIVITY OF NUCLEAR REACTORS SPECIFICATIONMy invention relates to device for controlling the reactivity of nuclearreactors.

It has been known heretofore to employ control rods with solidabsorption material that are stepwise insertable into and withdrawablefrom a reactor core for regulating the reactivity of a lightwater-cooled nuclear reactor, use being mostly made of a chemicaladditive, for example boron, in the coolant water. it has furthermorebeen proposed to employ in addition, individual finger control rodswhich are either inserted fully into the reactor core or are withdrawntherefrom in order to avoid axial irregularities or non-uniformities inthe power density distribution.

With such control devices, instabilities in the power densitydistribution during load changes, for example, in the course of a dailyload cyclic operation, of relatively large nuclear power plants cannotalways be avoided.

It is accordingly an object of my invention to provide device forcontrolling the reactivity of nuclear reactors which markedly controlssuch instabilities.

With the foregoing and other objects in view, I provide in accordancewith my invention device for controlling the reactivity of a nuclearreactor cooled by light water and including a reactor core formed offuel elements, comprising a tube system passing through the reactor coreand traversible by a water-soluble neutron absorber of adjustableconcentration, separated by the tube system from light water coolant inthe reactor core, the tube system comprising a plurality ofsubstantially vertically extending fingerlike tubes always fullyinserted in the fuel elements; a spiderlike holder holding thefingerlike tubes together at the upper ends thereof and serving asdistributor for the neutron absorber to the fingerlike tubes, and aholder rod extending from the spiderlike distributor and serving as acentral supply line for the neutron absorber to the spiderlikedistributor.

By employing groups of such fingerlike tubes, which serve as fingercontrol rods, a continuous variation in reactivity is rendered possible,the axial disturbance of the power density distribution being less thanfor the heretofore known control devices. Moreover, an increase in theman power density of about 25 percent is possible.

in accordance with other features of the invention, the fingerlike tubesof a unit or bundle thereof, are passed into respective guide tubes of afuel element and the bundle of fingerlilte tubes serving as control rodsare withdrawable as a unit during a fuel element exchange. The centralsupply line extends upwardly through an upper framework of the reactorcore and through the control rod tube of the cover of the reactorpressure vessel and is releasably sealed with respect to the vesselcover. Outer systems for variably adjusting the concentration of theneutron absorber are connected to the upper end of the central supplyline.

ln accordance with further features of the invention, the throughput orflow rate of absorber solution through the respective bundle offingerlike control tubes is such that heat produced by neutronabsorption, neutron braking or slowdown and y-absorption, is reliablyremoved thereby. The neutron absorber is, advantageously, an aqueoussolution of boric acid with an enrichment of B" with respect to thenatural boron content.

In accordance with yet another feature of the invention, and in order toreduce any danger of a possible leak in this tube system, means areprovided for maintaining the absorber solution under superpressurerelative to the, pressure of the reactor coolant.

According to an added feature of my invention, the fingerlike tubespassing through the reactor core are formed of a corrosion-resistant andneutron-permeable zirconium alloy, and are provided in the interiorthereof with axial supply tubes for the absorber solution.

According to additional features of the invention, units of thefingerlike tubes are held together in groups by the distributor spidersand respectively cover two to nine fuel elements of the reactor core,passing through the fuel elements, respectively, with at least onefingerlike tube. To permit simultaneous use of solid control rods andthe fingerlike control tubes, part of the fuel elements is not coveredby the units of fingerlike tubes and is provided instead, withconventional control rods containing solid absorber material for fullycutting off the reactivity or partial length control rods for affectingthe axial power density.

In accordance with other features of my invention and in order to permitthe insertion of conventional solid control rods, the spiderlikedistributors for the fingerlike tube units are located between the upperends of the fuel elements and an upper grid or core support plate of thecore framework, the solid control rods or partial length control rodsbeing insertable into the reactor core between the individual legs ofthe spiderlike distributor. The solid control rods being held togetherin groups at the upper ends thereof by actuating spiders guided in guideinserts, the actuating spiders being located above the spiderlikedistributors of the fingerlike tubes for the absorber solution even whenthe solid control rods are in fully inserted condition in the reactorcore.

In accordance with a concomitant feature of my invention, the fingerlikecontrol tube units are connected together by connecting lines for theabsorber solution in annular zones covering the cross section of thereactor core so that the radial power density distribution is able to becontrolled by variable adjustment of the concentration of the absorbersolution for the individual zones.

The fingerlike tube units of my invention, simultaneously offer thepossibility of attaching probes or sondes for instrumentation of thereactor core. Consequently, in accordance with another feature of myinvention, I provide a sonde in the form of a fingerlike tube which isdisposed in an extension of the central supply line so that the supplyfor the core instrumentation passes through the upper connection of thecentral supply line for the absorber solution and is releasablyconnected thereat with suitable cables or tube lines for withdrawing thesonde through the central supply line for absorber solution.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as device forcontrolling the reactivity of nuclear reactors, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of a reactor pressure vesselshowing the fingerlike control tubes, according to my invention, infully inserted position;

FIG. 2 is a much enlarged schematic view of a unit assembly of thefingerlike tubes of FIG. 1;

FIG. 3 is a plan view ofa fingerlike tube unit assembly having forexample nine groups each with four fingerlike tubes;

FIG. 4 is a diagrammatic view of the mosaic-like manner in which a fuelelement is covered with fingerlike control tube groups or bundles;

FIG. 5 is a partial enlarged diagrammatic elevational view of afingerlike tube unit assembly shown in a fuel element in a reactor andalso including auxiliary conventional solid control rods; and

FIG. 6 is a schematic view ofa fingerlike tube unit or bundle having anadditional fingerlike tube for nuclear core instrumentation.

Referring now to the drawings and first particularly to FIG. 1 thereof,there is shown a reactor pressure vessel 1 having a reactor core 2within which fuel elements 3 are disposed. The fuel elements 3 aresupported at the upper ends thereof by an upper core framework 4. Withina fuel element 3 extending along the axis of the pressure vessel 1 thereis disposed a fingerlike control tube unit or bundle 5 which includesthe individual fingerlike tubes 6. The fingerlike tubes 6 are suspendedfrom a spider like distributor 7, and a supply tube 8 extends axiallyfrom the distributor 7 upwardly through the upper core framework 4 andpasses through a cover seal 9 in the cover 10 of the reactor pressurevessel 1.

In the enlarged schematic view of the fingerlike control tube unit orbundle 5 shown in FIG. 2, the individual fingerlike tubes 6 are securedto the ends of the legs 11 of the spiderlike distributor 7. Theindividual fingerlike tubes 6 are formed of an outerjacket or encasingtube closed on the bottom thereof as well as an interior supply line 12of smaller diameter, for supplying absorber liquid, located coaxial tothe jacket. A supply tube 8 extends upwardly from the middle of thespiderlike distributor 7 and, in accordance with the embodimentillustrated in FIG. 2, two coaxial tubes 13 and 14 are located withinthe supply tube 8 and serve respectively for supplying and dischargingabsorber liquid. Within the spiderlike distributor 7, a line 15 branchesoff the inner supply tube 13 into each leg 11 of the spiderlikedistributor 7, the lines 15 being connected respectively with the supplylines 12 that extend into the fingerlike tubes 6. The absorber liquid isthus conducted through the supply tube 13 and the connecting orbranching line 15 through the individual coaxial supply lines 12 in thefingerlike tubes 6 until they reach the bottom of the jacket in theindividual tubes 6, and then flows upwardly again and through radialconnecting lines 16 disposed in the individual legs 11 of the spiderlike distributor 7 to the discharge tube 14 located within the commonsupply and holder tube 8.

The individual fingerlike control tube units 5 are supplied withabsorber liquid through upper connections 17, shown diagrammatically inFIG. 1, located at the end of the supply tube 8. Control of thethroughput or How rate and of the concentration is thus renderedpossible so that heat produced by neutron absorption, neutron braking orslowing down, or 'y absorption can be reliably removed. The absorbersolution is advantageously formed of an aqueous solution of boric acidwherein the boric acid is enriched with a quantity with a B in an amountgreater than the normal content of B in boric acid.

In the event of damage to the individual fingerlike control tubes orrods 6, it is advantageous to subject the absorber solution to a higheroperating pressure than that to which the reactor coolant is subjectedso that the neutron absorbing solution will flow into the reactorcoolant whenever a leak is formed within the reactor core. Thepossibility of such a disturbance causing reactivity to be freed, orrunaway reactivity is thereby reduced.

The bundle or unit 5 of fingerlike control tubes 6 are generallyconstructed so that several fuel elements 3, respectively, are coveredby one bundle 5 of control tubes 6. In FIG. 3, there is shown anarrangement wherein nine fuel elements 3 are covered by a single bundle5. Four fingerlike tubes 6 ofthe control tube bundle 5 extend into eachof the fuel elements 3 of FIG. 3. The construction of the spiderlikedistributor 7 for the case of the bundle 5 of FIG. 3 can be readily seenfrom this figure, the fingerlike control tube bundle having all told 36individual fingerlike tubes 6. It is also possible however that,respectively, only one, two or four fuel elements 3 are covered by onecontrol tube bundle 5.

In order however to permit simultaneously the insertion of conventionalsolid rods, partial length control rods or shut-off or scram rods, thefingerlike control tube groups traversed by absorber liquid, as shownschematically in FIG. 4, are disposed in such a mosaic-like manner thatwhen the reactor core is covered with groups of four liquid-filledcontrol tube units 5, respectively, one fuel element between these units5 is not covered so that a mechanically displaced solid conventionalcontrol rod or control rod finger 18 can be inserted therein. Thecoordination of mechanically displaced solid conventional control rods18 with liquid-filled control tube units 5 according to the invention isdisclosed in FIGv 5. A liquid-filled fingerlike control tube unit 5 withindividual fingerlike tubes 6 is inserted initially into the fuelelement 3 such a distance that the spiderlike distributor 7 is locatedbetween the upper ends of the fuel elements 3, i.e., above the fuelelement heads 19, and below the upper grid plate 20 of the nuclear corframework. In the intermediate space between the individualliquid-filled absorber tubes 6, a mechanically displaceable solidconventional control rod 18 with individual fingers 21 which are heldtogether by a spider 22 is inserted into the fuel elements 3. Theactuating spiders 22 of these mechanically displaceable control rods 18are located, in the inserted condition of the control rods 18, above thespiderlike distributor 7 of the liquid-filled fingerlike control tubeunits 5, and the individual absorber fingers 21 of solid control rod 18are located between the legs 11 of the spiderlike distributor-7 for theliquid-filled control tubes 6 so that the absorber fingers 21 of thesolid control rods 18 are able to be inserted and removed from the fuelelements respectively located thereunder, without any hindrance.

It is advantageous that the spiderlike distributor 7 of theliquid-filled control tube units 5 are so constructed that they fulfillsimultaneously the function of radially adjusting and axially holdingdown the fuel elements against the structures of the upper coreframework.

In order to control the radial power density distribution in the reactorcore 2, it is advantageous to combine the liquidfilled control tubegroups 5 in annular zones over the cross section of the reactor corewhereby separate adjustment of the concentration of the absorber liquidfor each zone is possible. The liquid-filled control tube units 5essentially serve to provide equalization or compensation for all rapidchanges in reactivity in the reactor core 2 such as power changes andchanges in fission product contamination. The axial power densitydistribution is controlled in a conventional manner by the partiallength control tubes 18 whereas the scramming or rapid shut-off iseffected by the conventional control rod fingers 21 having a solidabsorber. The reactivity change produced by burn-off is compensated forin a conventional manner by conforming the concentration of awater-soluble neutron absorber in the entire reactor coolant.

Such fingerlike control tubes 6 of my invention offer a good possibilityfor arranging an auxiliary probe or sonde for the core instrumentationas shown in FIG. 6. Thus, an additional fingerlike tube 23 is located inthe axis of the fingerlike con- -trol tube unit 5, the tube 23containing the core instrumentation. The instrumentation is built intothe tube 23 in such a manner that the supply lines 24 pass through thesupply tube 8 at the upper connection thereof and are releaseablyconnected thereat to the corresponding cables or tube lines. Theseprobes or sondes 23 can then, if desired, also be withdrawn through thecentral supply tube 8.

In order to remove these liquid-filled control tube units, all of theconnectors 17 and 24 at the upper end of the supply tube 8 are loosenedor released, the cover of the reactor pressure vessel and the upper coreframework located above the liquid-cooled fingerlike control units areremoved so that the latter can be withdrawn from the respective fuelelements by means of a guide tool while the fuel element exchange can becarried on or held in free suspension and then reinstituted after theexchange of the respective fuel element.

The aforedescribed new device for chemically controlling the reactivityin light water-cooled nuclear reactors is particularly reliable inoperation because it has no moving parts and no mechanisms within thereactor core. Moreover, less important and stable structures arepossible thereby in the upper core framework. Due to the relatively easycontrol of the throughput or flow rate and the concentration of theabsorber liquid, a continuous reactivity variation is possible whichdoes not effect any axial disturbance of the power density distribution.Moreover, due to the aforedescribed and illustrated arrangement, theheight of the installation at the upper core framework and at thereactor pressure vessel is minimized. The special advantage isparticularly that the variations in reactivity connected with the dailycyclical operation is controllable while avoiding unduly greatdisturbance of the power density distribution, relief of the boric acidcontrol system from rapid concentration changes being moreover possiblethereby.

lclaim:

1. Device for controlling the reactivity of a nuclear reactor cooled bylight water and including a reactor core formed of fuel elements,comprising a tube system passing through the reactor core andtraversible by a watersoluble neutron absorber of adjustableconcentration separated by the tube system from light water coolant inthe reactor core, said tube system including at least one control tubeunit comprising a plurality of substantially vertically extendingfingerlike tubes always fully inserted in the fuel elements duringoperation of the reactor; a spiderlike holder holding said fingerliketubes together at the upper ends thereof and serving as distributor forthe neutron absorber to said fingerlike tubes, and a holder rodextending from said spiderlike distributor and serving as a centralsupply line for the neutron absorber to said spiderlike distributor.

2. Device according to claim 1 wherein the fingerlike tubes of saidcontrol tube unit extend into a respective guide tube of a fuel element,and the fingerlike tubes of said control tube unit' are removabletogether when the fuel element is exchanged.

3. Device according to claim 1 wherein the reactor core has an upperframework situated therewith in a pressure vessel closed by an uppercover, wherein said holder rod supply line extends centrally throughsaid upper framework and through an opening formed in said pressurevessel cover, said opening being sealed between said holder rod supplyline and said pressure vessel cover.

4. Device according to claim 3 including means for connecting to theupper end of said supply line outer systems for variably adjusting theconcentration of the neutron absorber.

5. Device according to claim 1 including means for applying neutronabsorber solution at predetermined flow rate to said tube system forreliably removing heat produced by neutron absorption, neutron brakingand a-absorption.

6. Device according to claim 1 wherein said neutron absorber is anaqueous solution of boric acid having an enrichment of B with respect tothe natural boron content.

7. Device according to claim 1, including means for subjecting theneutron absorber solution to a higher pressure than that of the reactorcoolant water.

8. Device according to claim 1 wherein said fingerlike tubes are formedof a corrosion-resistant and neutron-permeable zirconium alloy andcontain in the interior thereof axially extending supply tubes for theneutron absorber solution.

9. Device according to claim 1 wherein a plurality of said control tubeunits are held together in groups by respective distributor spiders,each of said groups covering two to nine fuel elements of the reactorcore and passing through the respective fuel elements with at least onefingerlike tube.

10. Device according to claim 9 wherein said groups cover the reactorcore so as to leave part of the fuel elements thereof exposed, andincluding control rods containing solid absorber material insertableinto said exposed fuel elements.

11. Device according to claim 10 wherein said control rods are partiallength control rods for controlling axial power density ofthe fuelelements.

12. Device according to claim 1 wherein said spiderlike distributors forsaid control tube units are located between the upper ends of the fuelelements and an upper grid plate of a framework for the nuclear core,and including control rods containing solid absorber material insertableinto the reactor core between individual legs of said spiderlikedistributors, said control rods being he! together in groups at theupper ends thereof by respective actuating spiders guidable in guideinserts carried by the core framework, said actuating spiders beinglocated above said spiderlike distributors of said fingerlike tubes evenwhen said control rods are in fully inserted condition in the reactorcore.

13. Device according to claim 1 wherein a plurality of said control tubeunits are connected together by connecting lines for the absorbersolution in annular zones covering the cross section of the reactorcore, the concentration of absorber solution of the control tube unitsin the respective zones being variably adjustable for controlling radialpower density of the reactor core.

14. Device according to claim 1 wherein said control tube units areprovided, respectively, with additional individual fingerlike tubesbypassed by said absorber supplying spiderlike distributor and adaptedto receive therein sondes for core instrumentation.

15. Device according to claim 14 wherein an additional fingerlike tubeis disposed in an extension of said central supply line so that supplylines for said core instrumentation extend through an upper connectionof said central supply line, and said sondes are upwardly withdrawablethrough said central supply line.

1. Device for controlling the reactivity of a nuclear reactor cooled bylight water and including a reactor core formed of fuel elements,comprising a tube system passing through the reactor core andtraversible by a water-soluble neutron absorber of adjustableconcentration separated by the tube system from light water coolant inthe reactor core, said tube system including at least one control tubeunit comprising a plurality of substantially vertically extendingfinger-like tubes always fully inserted in the fuel elements duringoperation of the reactor; a spider-like holder holding said finger-liketubes together at the upper ends thereof and serving as distributor forthe neutron absorber to said finger-like tubes, and a holder rodextending from said spider-like distributor and serving as a centralsupply line for the neutron absorber to said spider-like distributor. 2.Device according to claim 1 wherein the finger-like tubes of saidcontrol tube unit extend into a respective guide tube of a fuel element,and the finger-like tubes of said control tube unit are removabletogether when the fuel element is exchanged.
 3. Device according toclaim 1 wherein the reactor core has an upper framework situatedtherewith in a pressure vessel closed by an upper cover, wherein saidhOlder rod supply line extends centrally through said upper frameworkand through an opening formed in said pressure vessel cover, saidopening being sealed between said holder rod supply line and saidpressure vessel cover.
 4. Device according to claim 3 including meansfor connecting to the upper end of said supply line outer systems forvariably adjusting the concentration of the neutron absorber.
 5. Deviceaccording to claim 1 including means for supplying neutron absorbersolution at predetermined flow rate to said tube system for reliablyremoving heat produced by neutron absorption, neutron braking and Alpha-absorption.
 6. Device according to claim 1 wherein said neutronabsorber is an aqueous solution of boric acid having an enrichment ofB10 with respect to the natural boron content.
 7. Device according toclaim 1, including means for subjecting the neutron absorber solution toa higher pressure than that of the reactor coolant water.
 8. Deviceaccording to claim 1 wherein said finger-like tubes are formed of acorrosion-resistant and neutron-permeable zirconium alloy and contain inthe interior thereof axially extending supply tubes for the neutronabsorber solution.
 9. Device according to claim 1 wherein a plurality ofsaid control tube units are held together in groups by respectivedistributor spiders, each of said groups covering two to nine fuelelements of the reactor core and passing through the respective fuelelements with at least one finger-like tube.
 10. Device according toclaim 9 wherein said groups cover the reactor core so as to leave partof the fuel elements thereof exposed, and including control rodscontaining solid absorber material insertable into said exposed fuelelements.
 11. Device according to claim 10 wherein said control rods arepartial length control rods for controlling axial power density of thefuel elements.
 12. Device according to claim 1 wherein said spider-likedistributors for said control tube units are located between the upperends of the fuel elements and an upper grid plate of a framework for thenuclear core, and including control rods containing solid absorbermaterial insertable into the reactor core between individual legs ofsaid spider-like distributors, said control rods being held together ingroups at the upper ends thereof by respective actuating spidersguidable in guide inserts carried by the core framework, said actuatingspiders being located above said spider-like distributors of saidfinger-like tubes even when said control rods are in fully insertedcondition in the reactor core.
 13. Device according to claim 1 wherein aplurality of said control tube units are connected together byconnecting lines for the absorber solution in annular zones covering thecross section of the reactor core, the concentration of absorbersolution of the control tube units in the respective zones beingvariably adjustable for controlling radial power density of the reactorcore.
 14. Device according to claim 1 wherein said control tube unitsare provided, respectively, with additional individual finger-like tubesbypassed by said absorber supplying spider-like distributor and adaptedto receive therein sondes for core instrumentation.
 15. Device accordingto claim 14 wherein an additional finger-like tube is disposed in anextension of said central supply line so that supply lines for said coreinstrumentation extend through an upper connection of said centralsupply line, and said sondes are upwardly withdrawable through saidcentral supply line.